1. Field of the Invention
The present invention relates to RF repeater arrangements for use in wireless telephone systems and, more particularly, for linking base stations to mobile wireless handsets in such systems, and is applicable to Time Division Duplex (TDD) signals and to Frequency Division Duplex (FDD) signals.
The invention may be used in wireless telephone systems using a signal conduit (e.g. co-axial cable, fibre optic cable, microwave links, infra-red links, cable TV plant or a combination of two or more thereof) to link a number of RF repeater elements, e.g. microcell extenders to a base station.
2. Description of Related Art
Base stations are employed to interface public switched telephone networks to RF signals, i.e. base stations transmit and receive RF signals to and from wireless telephony networks. Typically, a base station can support a number of simultaneous voice links.
Such base stations have RF signal transmitting and receiving equipment and control equipment and can be connected through a coaxial cable or other signal conduit to one or more RF repeaters, which interface with wireless handsets, i.e. broadcast transmit signals from the base station to the wireless handsets as radio signals and also receive radio signals from the handsets and pass them to the base stations. In this way, the RF repeaters can be utilized to increase substantially the area which can be served by one base station. It is in many cases advantageous to make such an RF repeater as an arrangement of two RF repeater parts or elements, i.e. a first part or base station extender which interfaces with the base station and a second part or microcell extender which interfaces with the handsets. These two parts may be physically separated from one another by a long distance, e.g. several kilometers, and connected by a signal conduit in the form of e.g. co-axial cable or optical fibre cable.
In practice, the second or handset part of the RF repeater arrangement is often one of a number of such handset interface parts provided at different locations and connected in common to the first or base station interface part. In this way, there is provided an RF repeater which enables a single base station to serve a number of different locations.
A problem in the present RF repeater technology is the need to provide timing and level adjustment information to the second parts of the RF repeater which takes account their unique placements in the signal conduit network. For example, if the second RF repeater part is interconnected by 100 meters of co-axial cable to the first RF repeater part, it perceives the signal level attenuated by the 100 meters of co-axial cable. However, the RF loss factor over this co-axial cable length will be quite different from that experienced by another second RF repeater part that is interconnected by 200 meters of co-axial cable. To be able to broadcast the correct signal level, it is necessary to determine and compensate for the RF loss factors unique to the respective second RF repeater parts.
This problem may be addressed in a number of different ways:
1. The gain of each second RF repeater part may be manually adjusted. This is unattractive in a large network and may be ineffective in any event since, unless the RF insertion loss is known, it is difficult to set the transmit power of the handset interface when a plurality of transmit signals are present. PA1 Additionally, when cable TV networks are used as signal conduits, the losses of subscriber taps and splitters form a part of the overall RF losses. Since the losses associated with such equipment usually occur in the homes of the subscribers, they cannot readily be determined from outside the homes. PA1 Also the gain required can vary as a function of time, temperature, etc. PA1 2. Pilot signals may be added at the first RF repeater part to allow a conventional Automatic Level Control. This approach provides a general solution, but is often unwelcome since the addition of pilot signals increases the likelihood of spurious signals being broadcast from the RF repeater. This is because the pilot frequency must be close to the RF signal frequency if it is to have the same loss on the signal conduit. PA1 In some signal conduits (e.g. cable TV networks), there are additional complications in trying to acquire an appropriate frequency band for the pilot signals. PA1 3. The base station RF transmit signal may be employed as a pilot signal, and a conventional Automatic Level Control system may be based on this. This approach is useful in some circumstances, most notably when the RF signal conforms to a single carrier Time Division Multiple Access (TDMA) format, but is quite ineffective when used with other formats, e.g. multiple carrier TDMA or Frequency Division Multiple Access (FDMA). PA1 4. Some RF signalling protocols (e.g. CT-2 Plus), have Control and Signalling Channels (CSCs) or similar beacons, that can be used as a level reference. Unfortunately, the CSCs were not defined for this purpose. They may become absent during a voice link, or they may change levels under adaptive power control environment in a fashion inconsistent with their use as a level reference.
In the above examples, the problem has been flamed in terms of the transmit power level. An identical problem exists in terms of the receive power level: a large array of second RF repeater parts works best if each second RF repeater has an identical receive path gain back to the base station, measured through its unique signal conduit interconnects.